1. Field of the Invention
This invention relates generally to improving the reliability of electrical components assembled on substrates, and more particularly to improving the reliability of electrical components, such as power semiconductors, in the presence of conductive contamination.
2. Description of the Prior Art
In many data processing systems (e.g., computer systems, programmable electronic systems, telecommunication switching systems, control systems, and so forth) electrical components (e.g., power semiconductors) are used in power units that supply hundreds or thousands of watts. Power factor correction is frequently required in such power units, and this is conventionally accomplished by inductively boosting the peak incoming line voltage above the peak voltage level seen on the power source. For example, in 240 volt alternating current (VAC) systems, 400 volts direct current (VDC) is routinely chosen as the boost potential. Electrical components, such as power semiconductors (e.g., field effect transistors and fast Schottkey diodes) provide efficient conversion to achieve such boost conversions.
The industry standard package styles for electrical components (e.g., power semiconductor devices, and other types of electronic devices) include several package styles (e.g., TO-220, TO-264, and TO-247). Such electrical components are typically encapsulated in a plastic body for through-hole lead attachment to a substrate. The body of an electrical component is frequently thermally coupled to a heat dissipation device (e.g., a heat sink, a heat-pipe, a fluid cooling system, a cooling fan, or other equivalent devices).
As more of these power units are shipped to customers, a serious reliability problem has emerged. Catastrophic failure involving electrical arcing (i.e., arc-over), and even fire in the power unit has become more common. The failure mode of the returned power units has been analyzed, and the source of failure has been found to be located in the electrical components (e.g., the power semiconductors). The initial source of failure and ignition is usually located in one of two places. Electrical arcing either occurs between adjacent leads of the power semiconductor packages where the leads connect to boost potentials, or the failure occurs between one of the leads and the heat dissipation device attached to the body of the power semiconductor. However, the spacing between adjacent leads or between any lead and the heat dissipation device is sufficiently large to withstand 400 volts of boost potential under normal conditions. The catalyst that initiates electrical arcing is the addition of conductive contaminants, such as “zinc whiskers” or other similar micro-conductors between adjacent leads or between a lead and the heat dissipation device.
Zinc whiskers are microscopic filaments of zinc metal that are prevalent wherever zinc plated metal is present. Zinc, being a sacrificial anode for steel, is used extensively as a plating layer wherever steel is used. Zinc can be found in air ducts, in cooling plenums used for cooling a data processing system, and even on the data processing system enclosure itself. Airborne zinc whiskers are plentiful wherever high air velocities are present. The same is true for other airborne metallic filaments, but zinc is more prevalent and more likely to form and sustain long filaments.
One of the reasons that power unit electrical arcing has become an increasingly serious reliability problem is that the airflows needed to keep these data processing systems cool are increasing as the power dissipation levels in data processing systems are increasing. The result is higher velocities of zinc-rich air across electrical components (e.g., power semiconductors) in need of cooling. These are the same electrical components most vulnerable to electrical arcing.
The process of electrical arcing begins with a tiny conductive path across a high potential (i.e., a high voltage). This requirement is satisfied with an accumulation of zinc whiskers joined with dust filaments on the leads of the power semiconductors or other electrical components. The result is a matrix of conductive material embedded in non-conductive dust. When a sufficiently small gap forms in the matrix, the 400 VDC boost potential will electrically arc across the gap. Zinc whiskers are generally not substantial enough to maintain the small gap necessary for 400 VDC to continue the electrical arcing. The tiny zinc whiskers rapidly vaporize, but in doing so, they ionize the surrounding air and provide a low impedance path in which the electrical arc is continued. The heat from the ion arc carbonizes the surround dust particles and/or the plastic semiconductor body, and provides a stable low impedance path resulting in a carbon flash. The resulting current spike destroys the nearest power semiconductor or other semiconductors, and causes the data processing system to fail.
Conventional solutions for this problem involve a variety of unattractive remedies, none of which is in widespread use. The most obvious conventional solution to reduce the electrical arcing is a new semiconductor design with much larger lead spacing. Unfortunately, this kind of packaging does not now exist, and would tax the power unit design with new constraints, such as increased size and more expensive heat dissipation device attachment. Furthermore, this approach would not eliminate the problem, but only reduce the severity and frequency of occurrence of the problem.
Another conventional solution is using a conformal coating on the substrate, such as a printed circuit board (PCB). A conformal coating normally includes dipping or spraying the substrate (e.g., a PCB) with an insulating paint or other polymer. While conformal coatings would solve the electrical arcing problem, the resulting assembly would be completely un-repairable and also un-recyclable. A conformal coating is not commonly applied to PCBs or other substrates produced in volume, because of the associated costs. The conformal coating process is expensive, and is also difficult to control on PCBs or other substrates assembled with connectors (e.g., connectors for cables).
An alternative conventional solution is a selective conformal coating, instead of a full conformal coating. A selective conformal coating typically involves a paste material selectively applied to the area of interest on a PCB or another substrate. In this case, the conformal coating covers the leads of the power semiconductors and other critical electrical components. While a selective conformal coating could be an effective solution for the described problem of electrical arcing, the application of a selective conformal coating is restricted to a manual, labor-intensive process. The possibility of misapplication resulting in incomplete coverage is substantial. Even in proper applications of a selective conformal coating, the process is time consuming, expensive, and messy.
It would be desirable to provide much of the protection of a fully conformal coating with a process that is inexpensive, manufacturing-friendly, and recyclable. What is needed is a consistent, particle-tight seal around each of the leads of electrical components (e.g., power semiconductors and other critical electrical components).